The study was led by sociologist Edward Laumann, considered a top authority on the sociology of sex, who believes the findings show that relationships based on equality lead to more satisfaction for both genders.

"Male-centered cultures where sexual behavior is more oriented toward procreation tend to discount the importance of sexual pleasure for women," Laumann said.

"When mama's not happy, nobody's happy," he said.

The study appears in the April issue of the Archives of Sexual behavior. It was funded by Pfizer, which makes the impotence drug Viagra.

Researchers surveyed 27,500 people by phone, in person or by mail, depending on local practices. The difference in questioning methods was one of the study's limitations, the researchers noted.

A nation's level of health and education could contribute to the findings, said John DeLamater, a professor at the University of Wisconsin and editor of the International Journal of Sex Research, who was not involved in the research.

"It's conceivable that people in developed countries have more information about sexuality. And they're also healthier," DeLamater said. "Being better informed, and being in better shape, they may be more able to maintain a satisfying sex life."